The present invention relates to a concatenated encoding arrangement suitable for space communications, and more particularly, to an inner code useful with a Reed-Solomon outer code in a concatenated encoding arrangement.
Referring to FIG. 1, known encoding arrangements 10 for use with satellite communication systems typically employed a concatenated coding scheme having a convolutional inner code sequence, and a Reed-Solomon (RS) outer code sequence as a way of providing high rate communication links at low signal power levels. Such systems include an RS encoder 12, a convolutional encoder 14, and modulator 16 at a transmitter 18. The convolutional code requires a Viterbi decoder 20 coupled to a demodulator 22 at the receiver end 24.
In addition, such a coding arrangement also requires an interleaver 26 between the convolutional and RS encoders to minimize the impact of burst errors. More specifically, in a concatenated coding system, an outer RS decoder 28 is undisturbed by errors which occur within a given eight bit symbol, i.e., about one constraint length of the convolutional inner code. However, performance of the RS decoder is degraded by longer burst errors, e.g., errors occurring among successive symbols, as can occur in the operation of the Viterbi decoder. As a consequence, interleaving the RS outer code is required for preventing or minimizing correlated errors, among successive symbols in the Viterbi inner decoder. Interleaving is defined as dispersing the original sequence of symbols in an RS codeword or frame in a block of frames by reordering the sequence so that consecutive bits of data are separated by a predetermined distance. A deinterleaver 30 is connected between the Viterbi decoder and the RS decoder.
While the use of an interleaver improves performance of a concatenated coding scheme using an RS outer code and a convolutional inner code, interleaving causes undesirably large processing delays for systems which use small packets either due to large minimum uplink burst size, or complex link protocols to pump or flush the interleaved data. In addition, such an arrangement requires prohibitively large on-board memory in systems where switching control necessitates on-board decoding.
It is therefore an object of the present invention to provide a coding scheme and communication system which utilizes concatenated coding without the need for interleaving the code sequence.
It is another object of the present invention to provide an inner coding scheme for use with an RS outer code in a concatenated coding system which does not require interleaving and is soft decision decodable.
In accordance with these and other objects, the present invention provides a system for transmitting concatenated encoded data signals having an inner code sequence and an outer code sequence, wherein the system includes a means for receiving a source signal having eight bit data symbols, a Reed-Solomon encoder for generating the outer code sequence based on the source signal, and an encoder for generating the inner code sequence as a the inner code sequence as a (n,k) short binary block code based on the source signal, wherein the short block encoder is arranged to generate the short block code with a message length k=8xc3x97i, with i being an integer small enough to obviate interleaving between the inner and outer codes. A transmitter processes and sends the inner and outer code sequences as the concatenated signal.
In accordance with a first embodiment of the present invention, the inner code comprises a (12,8;3) inner code formed from a punctured (15,11) Hamming code sequence. More specifically, three columns of a parity check matrix are selectively deleted to form the (12,8;3) inner code. The eight bit source signal would comprise a sequence of a0, a2, a3, a4, a5, a6, and a7, and the inner code would be formed from the sequence of a7, a6, a5, a4, a3, a2, a1, a0, P3, P2, P1, and P0, where:
P3=a1, XOR a2 XOR a3 XOR a4 XOR a5 XOR a6 XOR a7;
P2=a0 XOR a4 XOR a5 XOR a6 XOR a7;
P1=a0 XOR a2 XOR a3 XOR a6 XOR a7; and
P0=a0 XOR a1 XOR a3 XOR a5 XOR a7.
In accordance with a second embodiment, the inner code comprises a (12,8;3) inner code formed from two short block sequences. More specifically, a first block code sequence is formed from a subset of the eight data bits of the source signal to be transmitted and a parity bit based on the subset of data bits. A second block code sequence is formed from the remaining data bits of the source signal and three parity bits based on the remaining data bits. The short binary block code is formed as a sequence of both the first block code sequence and the first block code sequence combined with the second block code sequence. With both embodiments, the short block inner code is soft decision decodable at the receiving end.
In accordance with another aspect of the present invention, a method is provided for transmitting concatenated encoded data signals having an inner code sequence and an outer code sequence, wherein the method comprises receiving a source signal having eight bit data symbols, generating a Reed-Solomon encoded outer code sequence based on the source signal, and generating an inner code sequence as a (n,k) short binary block code based on the source signal, wherein the short block inner code is generated with a message length k=8xc3x97i, where i is an integer small enough to obviate interleaving between the inner and outer codes. The inner and outer code sequences are then processed and transmitted as a concatenated signal.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.